A US Air Force unmanned spacecraft has blasted off from Florida, amid a veil of secrecy about its military mission.
The robotic space plane, or X-37B, lifted off from Cape Canaveral atop an Atlas V rocket at 7:52pm local time on Thursday (0952 AEST on Friday), according video released by the military.
"The launch is a go," Air Force Major Angie Blair told AFP.
Resembling a miniature space shuttle, the plane is 8.9 meters long and has a wing-span of 4.5 meters.
The reusable space vehicle has been years in the making and the military has offered only vague explanations as to its purpose or role in the American military's arsenal.
The vehicle is designed to "provide an 'on-orbit laboratory' test environment to prove new technology and components before those technologies are committed to operational satellite programs," the Air Force said in a recent release.
Officials said the X-37B would eventually return for a landing at Vandenberg Air Force Base in California, but did not say how long the inaugural mission would last.
"In all honesty, we don't know when it's coming back," Gary Payton, deputy undersecretary for Air Force space programs, told reporters in a conference call this week.
Payton said the plane could stay in space for up to nine months.
Flight controllers plan to monitor the vehicle's guidance, navigation and control systems, but the Air Force has declined to discuss what the plane is carrying in its payload or what experiments are scheduled.
Pentagon officials have sidestepped questions about possible military missions for the spacecraft, as well as the precise budget for its development - estimated at hundreds of millions of dollars.
The results of the test flight will inform "development programs that will provide capabilities for our warfighters in the future," Payton said.
The space plane - manufactured by Boeing - began as a project of NASA in 1999, and was eventually handed over to the US Air Force Rapid Capabilities Office.
The Air Force has plans for a second X-37B, scheduled to launch in 2011.